1. Field of the Invention
The present invention relates generally to signal processing. More particularly, the present invention relates to speech signal processing.
2. Background Art
The VoIP (Voice over Internet Protocol) network is evolving to deliver better speech quality to end users by promoting and deploying wideband speech technology, which increases voice bandwidth by doubling sampling frequency from 8 kHz up to 16 kHz. This new sampling rate leads to include a new high band frequency up to 7.5 kHz (8 kHz theoretical) and will extend the speech low frequency region down to 50 Hz. This will result in an enhancement of speech naturalness, differentiation, nuance, and finally comfort. In other words, wideband speech allows more accuracy in hearing certain sounds, e.g. better hearing of fricative “s” and plosive “p”.
The main applications that are being targeted to take advantage of this new technology are voice calls and conferencing, and multimedia audio services. Wideband speech technology aims to reach higher voice quality than legacy Carrier Class voice services based on narrowband speech having sampling frequency of 8 kHz and a frequency range of 200 Hz to 3400 (4 kHz theoretical.) As the legacy narrowband phone terminals were prioritizing the understandability of speech, the new trend of wideband phone terminals will improve the speech comfort. Wideband speech technology is also named as “High Definition Voice” (HD Voice) in the art.
FIG. 1 shows speech frequency band 100, which provides for a comparison between the wideband voice frequency bandwidth and the legacy traditional narrowband voice frequency bandwidth. As shown, the wideband voice frequency bandwidth extends from 50 Hz to 7.5 kHz, whereas the legacy traditional narrowband voice frequency bandwidth extends from 200 Hz to 3.4 kHz.
However, before the wideband speech can be fully deployed in infrastructure as network and terminals, an intermediate narrowband/wideband co-existence period will have to take place. Experts estimate the transition period from wideband to narrowband may take as long as several years because of the slowness to upgrading the infrastructure equipment to support wideband speech. In order to improve the speech quality during this intermediate period or in systems where narrowband and wideband speech co-exist, some signal processing researchers have proposed several models, which are mostly based on an extension mode of CELP speech coding algorithm. Unfortunately, the proposed models suffer from consumption of high processing power, while providing a limited performance improvement.
Accordingly, there is a need in the art to address the intermediate period of narrowband/wideband co-existence, and to further improve speech quality for systems, where narrowband and wideband speech co-exist, in an efficient manner.